The following article is taken from the Compendium of Theology of St. Thomas Aquinas, section under 'Faith', 232.
It was one of the fundamental difficulties of the early Church to deal with who exactly Christ was. After all, our salvation depends on the one who saved us. Various heresies were produced that denied one aspect of Christ's person which ultimately affected the capacity of the Lord to be a competent saviour. Some claimed that He was only human, others only divine, others denied the full humanity, others the full divinity and others still the unity of His person. It is utterly vital for us to proclaim the full humanity, the full divinity (homoousios) and the unity of Christ in one subsistent hypostasis. The following article hinges on this belief and so does our salvation.
With the Angelic Doctor we acknowledge that the human body and soul form one substance. Men are not spirits trapped in a cruel but temporary prison. We are only fully human when both exist peacefully together. As the body suffers, the soul in some way experiences this harm. We cannot forget that Christ had a created soul and created body which did not exist prior to the Incarnation. So the following reflections on His suffering depend on understanding that due to the creation nature of His human body and soul (however pure) He was capable of feeling pain. Thomas tells us that the soul can experience two kinds of suffering, 1) from the body itself, and 2) from the object. He informs us that the whole soul suffers (due to the unity between them. the soul being the form of the body) because of the suffering of the body. It follows as well that since the powers of the soul are found in the soul's essence, each of its powers suffers when the body does.
However, we must make a vital qualification here when we consider the capacity of Christ's soul to suffer harm. We have to deny that the object of suffering could cause harm to every power of His soul. The reason for this is that Christ experienced the Beatific Vision in the higher part of His soul. It would have been impossible for some object to disturb this state and cause harm. The plenitude of glory was experienced in the higher part of His soul which nothing could have diminished. Thomas continues:
''Therefore, the higher reason of Christ's soul, which cleaves to the eternal things that should be contemplated and consulted, had nothing adverse or contrary whereby any suffering or harm would take place.''
So how is it that Christ could be said to suffer truly in soul if the higher part was incapable of being disturbed? We note that sense powers are vitally linked through their objects to sensible, created things, therefore pain can be transmitted to them.Strikes upon the body are felt through the nerves and the soul perceives these blows as harmful to the continuance of the whole person. As shown in the Garden, Christ was capable of suffering distress, when the knowledge of future things (the Crucifixion) caused Him internal distress and fear. The proximity of harm, and the certainty of it (He knew this in the Beatific vision and the knowledge of the mysteries that His Father infused, as well as being the Wisdom of God Himself) caused Him to sweat blood and shrink before death. The lower reason united with the sense powers perceives the harm being done to the body and proves itself adverse to it. Pain is regarded as contrary to the functioning of the body which wishes to preserve itself in tranquillity. Therefore we must admit that the suffering of the Cross proved 'odious' to His lower soul that wished to avoid the coming pain. It is for this reason that Christ asked the Father to remove the chalice of His passion from Him. Yet, we cannot forget His immediate response, 'Fiat voluntas tua!' We shall move onto an explanation of this shortly.
There are various kinds of suffering and one such sort involves suffering out of love of someone else. We may experience pain when a loved one commits some evil or goes through their own sickness or discomfort. It is completely nature for a human being to undergo this internal pain or confusion. Christ suffered this pain on the Cross when He considered all those for whom He died. As shown in a previous article on the knowledge of Christ, it was revealed to Him all those God has had created and would create, therefore Christ knew all those souls whose sins lead to His passion and death. He suffered when He regarded their sins and their punishments, however we must make another vital distinction here. The love of neighbour for God's sake belongs to higher reason. As said above, the higher part of His soul possessed the Beatific Vision so that knowledge of the sins of men and their punishments could not have disturbed Him. In the Beatific Vision, He, like all the saints, 'knew' how Divine Wisdom and Providence operates. They are capable of seeing how individual sins and damnations fit into the plan of God thereby they feel no distress. They can reconcile how goodness and justice form one whole in the deity.
In our case, we are unable to comprehend how the sins of our loved ones and their suffering can be reconciled with a loving God. On this account we feel distress and uncertainty, which did not plague Christ's higher reason which experienced the Beatific Vision. We cannot know of the eternal destinies of our friends and families, so we are told to pray and trust.
Could the suffering experienced by Christ's soul lead to division? We must answer no. When we wayfarers experience emotions arising from particular unpleasant situations our responses are often immoderate and disordered. We consider how we are inconvenienced and insulted instead of how God is offended. Our anger builds up contrary to right reason and we act badly as a result. However, in the case of Christ, this was not possible. The soul of our Lord was governed with rectitude. His emotions, which could only have come from a full and true humanity, were entirely subject to right reason and did not exceed their set boundaries. Thomas continues:
''Rather, the lower appetite, which is subject to emotions, was moved only as much as reason directed that it should be moved''
We must not admit any sort of disjunction in the soul of Christ was He was in possession of the Beatific Vision and He was free of any form of concupiscence. Although His soul suffered terribly from the pain inflicted on His sacred body and His soul perceived this as harm and His imagination suffered distress due to coming pain, His higher reason recognised why He had to suffer. He therefore in His higher reason willed to suffer pain for the sake of sinners, even while His lower reason linked to the senses and the awful torture inflicted on Him shrank from death. The latter shrank from death as is entirely natural, but the fear did not exceed its bounds.
In essence, the suffering did not destroy His enjoyment of the Beatific Vision nor vice versa, each part acted according to its proper function.